Throughout this application, various publications are referenced in parentheses by author and year. Full citations for these references may be found at the end of the specification immediately preceding the sequence listings and the claims. The disclosure of these publications in their entireties are hereby incorporated by reference into this application to describe more fully the art to which this invention pertains.
Neuroregulators comprise a diverse group of natural products that subserve or modulate communication in the nervous system. They include, but are not limited to, neuropeptides, amino acids, biogenic amines, lipids and lipid metabolites, and other metabolic byproducts. Many of these neuroregulator substances interact with specific cell surface receptors which transduce signals from the outside to the inside of the cell. G-protein coupled receptors (GPCRs) represent a major class of cell surface receptors with which many neurotransmitters interact to mediate their effects. GPCRs are predicted to have seven membrane-spanning domains and are coupled to their effectors via G-proteins linking receptor activation with intracellular biochemical sequelae such as stimulation of adenylyl cyclase.
Neuropeptide FF (NPFF) is an octapeptide isolated from bovine brain in 1985 by Yang and coworkers (1) using antibodies to the molluscan neuropeptide FMRFamide (FMRFa). FMRFamide-like immunoreactivity was observed in rat brain, spinal cord, and pituitary, suggesting the existence of mammalian homologs of the FMRFa family of invertebrate peptides. The isolation of NPFF, named for its N- and C-terminal phenylalanines (also called F8Famide) and a second mammalian peptide, NPAF (also called A18Famide), confirmed the existence of mammalian family of peptides sharing C-terminal sequence homology with FMRFa (1). Molecular cloning has revealed that NPFF and NPAF are encoded by the same gene and cleaved from a common precursor protein (2). Studies of the localization, radioligand binding, and function of NPFF-like peptides (see below) indicate they are neuromodulatory peptides whose effects are likely to be mediated by G protein-coupled receptors (for review, see 3).
NPFF, also called “morphine modulating peptide”, is an endogenous modulator of opioid systems with effects on morphine analgesia, tolerance, and withdrawal (for review see 3, 4). NPFF appears to represent an endogenous “anti-opioid” system in the CNS acting at specific, high-affinity receptors distinct from opiate receptors (5, 6). Endogenous NPFF has been suggested to play a role in morphine tolerance: agonists of NPFF precipitate “morphine abstinence syndrome” (i.e. symptoms of morphine withdrawal) in morphine-dependent animals (7, 8), while antagonists and anti-NPFF IgG restore morphine sensitivity and ameliorate symptoms of withdrawal (9–12). NPFF antagonists potentially could be useful as therapeutic agents to prevent the development of morphine tolerance, and to treat opiate addiction. NPFF has also been suggested to participate in the regulation of pain threshold, showing both “anti-opiate” effects and analgesic effects depending on test system and route of administration (for review, see 4). As an anti-opiate, NPFF has been shown to inhibit morphine- and stress-induced analgesia (1, 13, 14, 15), whereas anti-NPFF IgG (which blocks the biological activity of NPFF) potentiates these two phenomena (16, 17). An NPFF antagonist may be clinically useful in potentiating the analgesic effects of morphine, allowing use of lower doses without the development of tolerance. NPFF agonists may also exhibit analgesic activity in some model systems (14, 18, 19). The analgesia elicited by NPFF is typically sensitive to naloxone, indicating that it is mediated by release of endogenous opioid peptides (19, 20). The interaction of NPFF and opioid systems in regulating pain pathways is thus complex and may involve multiple mechanisms and sites of action. NPFF has additional biological activities in accord with its pattern of expression in the nervous system.
NPFF peptide localization in rat CNS was examined using specific antibodies ((21–23); see also (3)). The highest levels of NPFF are found in spinal cord and posterior pituitary; pituitary NPFF is believed to originate in the hypothalamus. In the brain, immunoreactive cell bodies are found in two major cell groups: medial hypothalamus (between dorsomedial and ventromedial) and nucleus of the solitary tract. Immunoreactive fibers are observed in lateral septal nucleus, amygdala, hypothalamus, nucleus of solitary tract, ventral medulla, trigeminal complex, and dorsal horn of spinal cord. This localization pattern is consistent with a role for NPFF in sensory processing and modulation of opioid systems. In addition, its presence in the hypothalamus and other limbic structures could subserve roles in the regulation of appetitive and affective states. In the periphery, NPFF-like immunoreactivity (as well as NPFF binding) has been observed in the heart (24). In addition, injection of NPFF raises blood pressure in rats (24, 25). These observations, combined with the colocalization of NPFF with catecholaminergic neurons in the nucleus of the solitary tract (26), suggest that NPFF is involved in central and peripheral cardiovascular regulation.
The ability of NPFF peptides to modulate the opioid system raised the possibility that NPFF interacts directly with opiate receptors. However, radioligand binding assays using a tyrosine-substituted NPFF analog [125I]Y8Fa demonstrate that NPFF acts through specific high affinity binding sites distinct from opiate receptors (27–30) that are sensitive to inhibition by guanine nucleotides (31). The latter observation indicates that NPFF receptors are likely to belong to the superfamily of G protein-coupled receptors which share common structural motifs. However, no reports of cloning NPFF receptors have appeared as yet.
To address the issue of potential degradation of the peptide radioligand, a more stable NPFF analog (called (1DMe)Y8Fa(18)) has also been radioiodinated and the binding characterized in spinal cord membranes (32). The binding was saturable and of high affinity; inhibition of binding with unlabeled NPFF analogs yielded Ki values of 0.16 nM and 0.29 nM for (1DMe)Y8Fa and NPFF, respectively, with a Bmax=15 fmol/mg protein. No inhibition by various opioid compounds (naloxone, morphine, enkephalins, dynorphins, etc.) or other peptides (NPY, SP, CGRP, for examples) was observed at a concentration of 10 μM, confirming the specificity of NPFF receptors. Interestingly, the related molluscan peptide FMRFa inhibited the binding of [125I](1DMe)Y8Fa with a Ki=30 nM. The effectiveness of FMRFamide and the C-terminal fragment NPFF (6–8) at NPFF receptors suggests an important role for the common C-terminus. Full activity is retained by NPFF (3–8); it has been suggested that although the C-terminus is important for receptor recognition, the N-terminus is necessary for formation of a high-affinity conformation (33).
Allard et al. (29) examined the distribution of NPFF binding sites in rat brain and spinal cord using [125I]Y8Fa ([125I]YLFQPQRFamide). The highest densities were observed in the external layers of dorsal horn of spinal cord, several brainstem nuclei, the suprachiasmatic nucleus, restricted nuclei of the thalamus, and the presubiculum of the hippocampus. Lower densities were seen in central gray, reticular formation, ventral tegmental area, lateral and anterior hypothalamus, medial preoptic area, lateral septum, the head of caudate-putamen and cingulate cortex. No binding was observed in cortex, nucleus accumbens, hippocampus (except in presubiculum) or cerebellum. The localization of NPFF binding sites is in good agreement with the location of the peptide itself, consistent with the binding sites mediating the biological actions of NPFF in these tissues (29, 34, 35). Less is known about the signal transduction pathways activated by NPFF receptors; NPFF was shown to activate adenylyl cyclase in mouse olfactory bulb membranes (36) but no other reports of functional coupling via G proteins have appeared.
Until now, no direct evidence for NPFF receptor subtypes has been reported in mammals. Recent physiological data suggest complex (biphasic) effects on nociception and antiopiate activity of NPFF (for review, see (3, 4)) that could possibly signal the presence of multiple subtypes. Short term ICV injection of NPFF causes a hyperesthesic effect followed by long lasting analgesic effect. Intrathecal NPFF and FMRFa both produce long-lasting analgesia, but subeffective doses caused different modulatory effects on morphine-induced analgesia (F8Fa potentiated, FMRFa decreased). The analgesic effects of NPFF are sensitive to naloxone, suggesting that NPFF receptors may have distinct presynaptic (increase release of opioids?) and postsynaptic (anti-opiate?) effects mediated by multiple receptors. Little is known of the biological effects of A18Famide, which shares its C-terminal 4 amino acids with NPFF, but the existence of a family of related peptides often is predictive of multiple receptor subtypes.
No nonpeptide agonists or antagonists of NPFF are available, but several useful peptidic analogs have been developed that exhibit increased agonist stability or antagonist activity. For example, desamino Y8Fa (daY8Fa) can antagonize the behavioral effects of NPFF and restore morphine-sensitivity (tail-flick analgesia) to morphine-tolerant rats at lower doses, although at higher doses it can act as NPFF agonist (10)(see also (3)). (1DMe)Y8Fa, in which L-Phe1 is replaced by D-Tyr and the second peptidic bond is N-methylated, has been shown to inhibit morphine-induced analgesia (18), and has higher affinity and stability than Y8Fa: (1DMe)Y8Fa was 90% stable after 150 min. incubation with rat spinal cord membranes compared with Y8Fa, which was fully degraded after 30 minutes. These analogs may be useful in predicting the effects of agonist or antagonist drugs that would act at NPFF receptors.
Despite the numerous studies linking NPFF with analgesia (for review, see (4)), only recently has NPFF been observed to play a role in animal models of chronic pain. For example, NPFF has recently been shown to be involved in inflammatory pain (37) and neuropathic pain (38). Importantly, NPFF was shown to attenuate the allodynia associated with neuropathic pain, suggesting that it may be clinically useful in treating this condition. In addition to its potential therapeutic roles in the treatment of pain and morphine tolerance ((4) and above) NPFF and related peptides have a number of other biological activities that may be therapeutically relevant including effects on feeding (39–41), psychotic behavior (42), nicotine addiction (43), and cardiovascular functions (24, 25). The cloning of NPFF receptors will facilitate the elucidation of the roles of NPFF and related peptides in these and other important biological functions.
Described herein is the isolation and characterization of a new family of neuropeptide FF (NPFF) receptors, referred to herein as the NPFF receptors. Cloned NPFF receptors will serve as invaluable tools for drug design for pathophysiological conditions such as memory loss, affective disorders, schizophrenia, pain, hypertension, locomotor problems, circadian rhythm disorders, eating/body weight disorders, sexual/reproductive disorders, nasal congestion, diarrhea, gastrointestinal, and cardiovascular disorders.